1. Field of the Invention
The present invention relates to a novel polymer gel composition capable of changing its volume by absorbing or releasing a liquid in response to an external stimulus and further relates to an optical device using such a polymer gel composition.
2. Description of the Related Art
In recent years, reports have been given regarding stimuli-responsive gels, which exhibit an expanding or shrinking phenomenon in response to various types of external stimuli. For example, such gels have a potential to be applied to a variety of fields such as a drug delivery systems, chemical actuators (such as artificial muscles and micro valves) and material separation. An optical device technology is also known in which stimuli-responsive polymer gels can be used to perform light modulation or fading, coloring by controlling transmittance or scattering of light.
Some stimuli-responsive gels are known to utilize an interaction of hydrogen bonds and ionic bonds and the like between polymer chains. Such gels use, for example, materials that form a polymer complex in a solution by hydrogen bond formation, such as a combination of poly(acrylic acid) and polyacrylamide. At low temperatures, poly(acrylic acid) and polyacrylamide can form a water-insoluble polymer complex with hydrogen bonds in water, but at high temperatures, the hydrogen bond is cleaved and the polymer complex can be dissolved in water. It is known that, if such polymers capable of forming a polymer complex are crosslinked so as not to dissolve in water, then such crosslinked materials can form a gel capable of changing its volume in response to temperatures, wherein the polymer chains in water can aggregate and shrink at low temperature and at high temperatures the hydrogen bond can be cleaved and the polymer chains expanded.
Such a gel utilizing the interaction between the polymer chains is known to change its volume in response to temperatures, pH, or solvent compositions. In particular, that the temperature-responsive interaction between polymers can produce low temperature-shrinking and high temperature-expanding characteristics, and some applications of such a material to a drug delivery system, has been proposed. For example, a hydrogen bond-utilizing system has been reported, which uses an interpenetrating network (IPN) structure of polyacrylamide and poly(acrylic acid), and investigations are being made regarding its application to control of drug containment/release properties (for example, see Japanese Patent Application Laid-Open (JP-A) No. 3-79068 and “J. Control Release” No. 16, pages 215-227 (1992).
However, the process of forming the IPN gel having a complex of two polymer gel components requires mixing a previously synthesized first component-containing polymer gel with a second component and allowing the components to penetrate before polymerization. Additionally, in order to process the gel particles into a particle shape, a certain technique, such as a method of performing the reaction in a very small vessel, is required to prevent the polymerization of the second component from producing the coupling between the first component particles. For example, a known method includes forming the first component-containing polymer gel in a capillary (for example, see Ilmain et al., Nature, 349, page 400, 1991). Another known method includes processing the first component-containing polymer gel into the desired form, then allowing the second component-containing solution to penetrate, and independently polymerizing the second component-containing polymer gel so that the first component-containing polymer gel particles do not aggregate by the second component (for example, see “J. Control Release” No. 16, pages 215-227 (1991). Another method includes allowing a monomer for forming the second component to penetrate into a large bulk gel that contains the first component, then performing polymerization and pulverizing the product. However, such a method is not industrially practical, since an extremely long time is necessary for the second component-forming monomer and crosslinker to penetrate into the first component-containing bulky gel.
As mentioned above, the conventional process of producing the IPN structure is generally complicated and can involve a very high cost particularly in cases in which particles or the like need to be processed. Thus, the conventional process is not suitable for mass production.